The present invention relates to an improvement in bearings for use in a screw compressor.
In general, the arrangement of a screw compressor is such that a pair of male and female rotors rotate in meshing relation to each other through teeth and grooves to compress gas such as refrigerant gas or the like. Because of such arrangement, large force acts upon the rotors, and the rotors are required to rotate at a high speed. The screw compressor is provided with radial bearings for supporting radial force from the rotors and thrust bearings for supporting axial force from the rotors.
Conventionally, angular-contact rolling bearings have been employed for the purposes described above, and have been arranged in plural to increase the load capacity. As a result, the bearings occupy much space so that the entire compressor tends to be brought to a large size. In addition, in a hermetic screw compressor, a semipermanent service life is required for each bearing. The rolling bearing is advantageous in that it is hard to seize up even under inferior lubricating conditions. However, the rolling bearing have a relatively short service life because of rolling fatigue.
Screw compressors employing rolling bearings are described, for example, in Japanese Patent Application Laid-Open Nos. 57-119191 and 57-206791.
In order to solve the problems discussed above, it is desirable that slide bearings or plain bearings are employed as a substitute for the screw compressor in rolling bearings. The plain bearing is advantageous in that the structure is compact, the cost is low, the service life can be prolonged, and the like. On the other hand, the plain bearing is subject to seizing and becomes unable to rotate, if formation of a lubricating oil film is insufficient.
Gas within the screw compressor is brought to a high temperature level in the course of compression of the gas, so that the lubricating oil often reaches a temperature level within a range of from 130 to 140 degrees C. Further, the lubricating oil is mixed with the compressed gas such as refrigerant or the like and tends to be reduced in viscosity, and the load applied to the bearings is high. In particular, if plain bearings are employed in a low-capacity hermetic screw compressor which utilizes a differential-pressure oil supply system, some of the plain bearings would seize up in a moment unless a sufficient amount of lubricating oil is fed to all of the plurality of bearings.
Japanese Patent Application Laid-Open No. 56-9694 discusses problems concerning the plain bearing lubrication in a screw compressor employing the differential-pressure oil supply system, and has proposed one solution to of the problems. According to this prior art, the lubricating oil is fed to a substantially central portion of each of two radial bearings for each of two rotor shafts. At the same time, in order to secure lubrication of each thrust bearing for the rotor shaft, the lubricating oil is supplied to a hydraulic pressure chamber defined between each thrust bearing and the adjacent radial bearing, through a passageway having provided therein a restriction, and the lubricating oil is fed from the hydraulic pressure chamber to the thrust bearing.
The solution proposed in the above prior art can get considerable results. If consideration is made to the inferior lubricating conditions as described above, however, it is desirable to further enhance the lubricating ability, in particular, of the thrust bearings. A plain bearing of dynamic-pressure type in which pressure is applied to the lubricating oil at a sliding section is superior in the ability of forming an oil film, and is effective for this purpose. However, supply of the lubricating oil according to the aforesaid prior art is inadequate for the case of employment of dynamic-pressure type thrust bearings, since supply of the lubricating oil to each thrust bearing and supply of the lubricating oil to the radial bearings are brought to an extremely unbalanced state.
That is, in case where the dynamic-pressure type thrust bearings are employed in a screw compressor and the lubricating-oil supply means as disclosed in Japanese Patent Application Laid-Open No. 56-9694 an amount of the lubricating oil passing through each thrust plain bearing is low, because the resistance to flow of the lubricating oil through the thrust plain bearing is high. On the other hand, the path guiding the lubricating oil to the central portion of the radial plain bearing adjacent to each thrust plain bearing is low in flow resistance. Accordingly, the lubricating oil almost does not flow to a section of the radial plain bearing on the side of the thrust bearing, but only a section of the radial plain bearing on the side opposite to the thrust bearing is lubricated. For this reason, the section of the radial plain bearing on the side of the thrust bearing seizes in a short period of time.